Of the various groups of the order _Rodentia_ found in South America, the _Sciuridæ_, so far as I am aware, are chiefly confined to the more northern parts, and do not occur in the most southern; the _Myoxidæ_, _Gerboidæ_, and _Arvicolidæ_ are wanting. The species of the family _Muridæ_ belong to different sections to those of the Old World. Of the _Leporidæ_ I am acquainted only with one well established species—the _Lepus Braziliensis_, which however is not found “in tota America Australi,” as Fischer says, there being no Hare yet found in the more southern parts, where the _Cavies_ and _Chinchillas_ appear to take their place. The remaining South American Rodents—certain species of _Hystricidæ_, the genera, _Echimys_, _Dasyprocta_, _Cælogenys_ and _Myopotamus_, together with the _Octodontidæ_ and _Chinchillidæ_, all possess a peculiar form of skull and of the lower jaw, (more or less approaching to figs. 1, Plate 33, and figs. 23, Plate 34.) which I have described in the “Magazine of Natural History,” for February 1839, and which is rarely found in the North American, or Old World Rodents. In enumerating the above groups, I omitted the _Caviidæ_, because in the form of the lower jaw they differ somewhat from the rest—they possess, in fact, a form of lower jaw peculiar to themselves; but in the Chinchillas[32] the transitions between one form and the other are found. The South American _Muridæ_, which form the chief part of Mr. Darwin’s collection, were none of them procured further north than latitude 30°, with the exception of those from the Galapagos Archipelago. The species occur at the following localities. WEST COAST OF SOUTH AMERICA. GALAPAGOS ARCHIPELAGO. Mus Jacobiæ. —— Galapagoensis. COQUIMBO. Mus longipilis. —— Renggeri. —— Darwinii. VALPARAISO. Mus Renggeri. —— _decumanus_. CONCEPCION. Mus longicaudatus. CHILOE AND CHONOS ARCHIPELAGO. Mus brachiotis. EAST COAST OF SOUTH AMERICA. MALDONADO. Mus _decumanus_. —— maurus. —— _Musculus_. —— tumidus. —— nasutus. —— obscurus. —— arenicola. —— bimaculatus. —— flavescens. Reithrodon typicus. BUENOS AYRES. —— Mus _decumanus_. BAHIA BLANCA. Mus Braziliensis. —— elegans. —— gracilipes. RIO NEGRO. Mus griseo-flavus. PORT DESIRE.